1. Field of the Invention
This invention relates to a light modulating device for use as a display panel, and more particularly, to a light modulating device which is suitable for a liquid crystal display medium.
2. Description of the Prior Art
In general, driving methods for display devices using liquid crystal material, electroluminescence (termed hereinafter "EL") and the like, can be classified into the static driving method and the multiplex driving method, according to the method of supplying signals.
In the static driving method, signals for display or erasing are applied independently to each display segment (picture element) electrodes. Accordingly, as the amount of display information (i.e., the number of display segments) is increased, the number of components, such as electrode terminals, connectors, interconnections, driving circuits or the like, is also proportionately increased. This increases the size and the cost of the entire display device. Hence, this driving method is limited to smaller applications such as clocks, bar graph displays, oscilloscope displays and the like.
On the contrary, the multiplex driving method is a method for increasing display capacity using fewer driving circuit elements by performing matrix display. In this method, all picture elements are divided into plural groups. Display frame time is also divided, and a display signal is applied to each group of picture elements in phase with the divided timing. Hence, this driving method is also called the time sharing driving method. Several techniques are considered in the method for dividing data signal and in the applied wave form. In the liquid crystal display device, for example, there have been devised the voltage averaging method, memory-effect-type driving method, double-channel driving method and the like. In the EL display device, there have been devised the field-refreshing driving method, p-n symmetrical driving method, push-pull symmetrical driving method and the like. In these techniques, however, when the number of picture elements is increased, the divided number of frame time also proportionately increases. Hence, the duty ratio (that is, the ratio of time allocated to each one of scanning lines to the frame time during the scanning of display signals) becomes small, and thus, the driving margin (the ratio of the effective-value voltage applied to the selected (lighting) picture element for display to that applied to the nonselected (nonlighting) picture element) is also decreased. Thus, driving becomes difficult.
Now, as a method for solving such problems, the active matrix driving method is attracting attention. This driving method is not characterized in its driving wave form, but in providing a switching device for each picture element. As a result, the duty ratio is 100%, and the driving margin can also be increased. However, although the active matrix driving method very effectively displays a large quantity of display information at a high quality, the driving method has a disadvantage in that it is necessary to provide a switching device for each intersection point (picture element) and thus its configuration becomes very complicated. As such switching devices, it has been considered to use the MIM (Metal-Insulator-Metal) devices, varistors, two-terminal ferroelectric devices, ring diodes and the like, and three-terminal devices made of CdSe, Te, a-Si, p-Si and the like. However, irrespective of the switching device which is used, it is impossible to avoid either the device configuration from being complicated or the difficult production process arising therefrom.